1. Field of the Invention
The invention described herein relates to an improved vibrating wire stress meter and a method of clamping the vibratory wire used.
2. Description of the Prior Art
Vibrating wire stress meters have been used for some time to measure the stress in rocks or other structures by noting variations in the vibrations of the is tensioned wires. Three examples of the prior art devices can be found in the U.S. Pat. No. 2,306,137 to W. Pabst et al, U.S. Pat. No. 2,969,677 to A. V. Lewis, and U.S. Pat. No. 3,675,474 to R. D. Browne. In many of these inventions the the wire tensioned in a longitudinal tube lengthwise of the tube to measure compressive forces on the tube ends. This type of arrangement along with the readout equipment needed to determine the frequency of vibration of the wire is usually expensive to fabricate and requires a knowledge of the modulus of the surrounding rock. My improved stress meter tensions the wire generally perpendicular of the tube's length to provide a unidirectional measurement of the compressive forces on the tube's outer surface. This construction allows the fabrication of a very stable, sensitive, simple, inexpensive, and rugged gauge whose operation is appreciably unaffected by changes in the surrounding borehole rock modulus. It does this without relying on a voltage source or value of the resistance from a connecting cable.
Another problem encountered in constructing vibrating wire stress meters is firmly anchoring the tensioned wire at its two ends. To allow for accurate readings in the vibrations it is necessary to precisely tension the wire. My invention not only provides for a precise tensioning of the wire but does so with a minimum of machining of the cylinder body. It also provides a water tight seal to the environment. As such it eliminates the use of screws, and the like, as anchoring devices which suffer from lack of accuracy and water tightness as well as requiring space for screw clamps.